Laser Photodetachment Research Articles

Dissociative Electron Attachment to NO, CO and N2O in a 0.5 T Magnetic Field

Dissociative electron attachment to CO, NO and N2O is studied in a 0.5 T magnetic field. The magnetic field dependence of the attachment process is analyzed to understand the possible role of magnetically-coupled excited molecular states. The present technique converts the O−(2P) ions produced by the attachment to fast O(3P) by steps of ion acceleration and laser photodetachment. The resulting fast O(2P) atoms are then detected with a microchannel plate. This method facilitates analysis of any B-field enhancements in the resonant attachment phenomena, with minimal contribution from backgrounds of positive and negative charges.

Observations of bound and unbound states of Ce−

The negative ion of cerium has been investigated with tunable infrared laser photodetachment spectroscopy over selected photon energy ranges between 0.56 − 0.70 eV. The spectrum reveals several sharp peaks due to negative ion resonances and possible bound-bound transitions in Ce−. The newly observed transitions, together with our previous measurements [1], provide insight into the rich near-threshold spectrum of this lanthanide negative ion.

Dynamics and Electronegativity of Oxygen RF Plasmas

AbstractThe capacitively coupled radio frequency plasma at 13.56 MHz in oxygen was systematically studied by 160 GHz Gaussian beam microwave interferometry at high temporal resolution (200 ns) and simultaneous laser photodetachment for electron and negative ion density analysis. Additionally, spatio‐temporally resolved electric probe measurements were performed for comparison with microwave interferometry. A high and low electronegative operation mode was found in the asymmetric rf discharge. In the high electronegative mode it was shown the significant role of the metastable excited oxygen molecules in electron attachment and detachment processes. In particular, a temporary electron density increase is observed in the early afterglow of a pulsed rf plasma. The transition between both modes is driven by the rf power and the self‐bias voltage, respectively. In connection with the phase resolved optical emission spectroscopy and the study of the electron heating mechanisms the transition into the low electronegative mode at higher rf power shows a relation to the alpha to gamma mode transition. Furthermore, electron density fluctuations are measured over a wide field of processing parameters, e.g. due to the attachment‐induced ionization instability. PIC‐MCC simulation and fluid model calculation of a symmetric oxygen rf discharge confirm the different electron heating mechanisms and the dominance of negative atomic oxygen ions (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron and Negative Ion Analysis in Oxygen Capacitively Coupled Radio Frequency Plasma

AbstractLow pressure capacitively coupled radio frequency plasmas (cc‐rf plasmas) were investigated by minimalinvasive 160 GHz Gaussian beam microwave interferometry. The interferometer is a frequency stabilized (PLL) heterodyne system which immediately provides the line integrated electron density at spatial (axial) resolution of about 10 mm and temporal resolution of 200 ns. The line integrated electron density was measured in dependence on the processing parameters rf power (10 … 100 W) and total pressure (10 … 100 Pa) in comparison between argon and oxygen. Thereby, the changed distance between the fixed microwave beam axis as well as the sheath edge is taken into consideration using an analytical collision‐dominated sheath model and results from optical emission spectroscopy. Microwave, interferometry and simultaneous laser photodetachment in oxygen plasma additionally permits the measurement of the negative atomic oxygen ion density. Dynamic and transient phenomena in oxygen plasmas are discussed such as mode transitions, afterglow behaviour in pulsed rf plasma, and electron density fluctuations due to attachment‐induced ionization instabilities (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser measurement of H– ions in a field-effect-transistor based radio frequency ion source

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.

Experimental and theoretical study of bound and quasibound states ofCe−

The negative ion of cerium is investigated experimentally with tunable infrared laser photodetachment spectroscopy and theoretically with relativistic configuration interaction in the continuum formalism. The relative cross section for neutral atom production is measured with a crossed ion-beam--laser-beam apparatus over the photon energy range of 0.54--0.75 eV. A rich resonance spectrum is revealed near the threshold with, at least, 12 peaks observed due to transitions from bound states of ${\mathrm{Ce}}^{\ensuremath{-}}$ to either bound or quasibound excited states of the negative ion. Theoretical calculations of the photodetachment cross sections enable identification of the transitions responsible for the measured peaks. Two of the peaks are due to electric dipole-allowed bound-bound transitions in ${\mathrm{Ce}}^{\ensuremath{-}}$, making cerium only the second atomic negative ion that has been demonstrated to support multiple bound states of opposite parity. In addition, combining the experimental data with the theoretical analysis determines the electron affinity of cerium to be 0.628(10) eV and the fine structure splitting of the ground state of ${\mathrm{Ce}}^{\ensuremath{-}}$ (${}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{7/2}--{}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{9/2}$) to be 0.097 75(4) eV.

The temporal evolution in plasma potential during laser photo-detachment used to diagnose electronegative plasma

A floating emissive probe is applied in conjunction with pulsed laser photo-detachment of O− ions to enable measurement of the dynamic evolution in a plasma potential resulting from the presence of photoelectrons in a 13.56 MHz inductive radio-frequency oxygen discharge. The emissive probe emits thermionic electrons, allowing it to reach a saturation potential which is characterized as the local space potential of the plasma. After the photo-detachment pulse, the local space plasma potential in the illuminated region shoots up to a higher positive value and then relaxes to equilibrium in microsecond time scales. Using the relaxation time of the space potential, the negative ion temperature of O− is estimated over a 10–50 mTorr range and is found to be in the 0.19–0.03 eV range. The negative ion temperature measured by this method is found to be lower than that calculated from the time evolution in electron density resulting from photo-detachment which is independently measured using a resonance hairpin probe.

Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

The time-resolved negative oxygen ion density n− close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O− density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m−3; however, in the off-time, the O− density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n−/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative electron attachment of highly excited oxygen molecules in the cooling plasma. The results show that close to the magnetic null the photodetached electron density decays quickly after the laser pulse, followed by a slow decay over a few microseconds governed by the negative ion temperature. However, in the magnetized regions of the plasma, this decay is more gradual. This is attributed to the different cross-field transport rates for electrons in these two regions. The resonance hairpin probe measurements of the photoelectron densities are compared directly to photoelectron currents obtained using a conventional Langmuir probe. There is good agreement in the general trends, particularly in the off-time.

The temporal evolution of the negative ion density in a low duty cycle pulsed reactive magnetron discharge

Time-resolved O− density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W.For all discharge powers, both the electron (ne) and negative ion (n−) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time ~25μs) for the first 50μs, followed by a slower rate (~150μs) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about ~150μs after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5×1016m−3 for a peak power of 720W, corresponding to a negative ion-to-electron density ratio n−/ne (α) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative.

A study of the plasma electronegativity in an argon–oxygen pulsed-dc sputter magnetron

Using Langmuir probe-assisted laser photodetachment, the temporal evolution of the O− density was determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon–oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5% to 50% and oxygen partial pressures of 10% and 50% of the total pressure.In the plasma on-time, for all duty cycles the negative ion density (n−) rises marginally reaching values typically less than 2 × 1015 m−3 with negative ion-to-electron density ratios, α < 1. However, immediately after the transition from pulse on-to-off, n− falls by about 20–30% as fast O− species created at the cathode exit the system. This is followed by a rapid rise in n− to values at least 2 or 3 times that in the on-time. The rate of rise of n− and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several tens of μs), and therefore the afterglow plasma becomes highly electronegative, with α reaching 4.6 and 14.4 for 10% and 50% oxygen partial pressure, respectively. The rapid rise in n− in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly excited oxygen metastables, which themselves are created in the pulse on-time. At the lowest duty of 5%, the long-term O− decay times are several hundred μs.Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, α has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method.

A novel approach for negative ion analysis using 160 GHz microwave interferometry and laser photodetachment in oxygen cc-rf plasmas

Microwave interferometry at 160.28 GHz with Gaussian beam propagation (beam waist: 5 mm) and laser photodetachment were combined for the analysis of negative atomic oxygen ions in the bulk plasma of an asymmetric capacitively coupled 13.56 MHz discharge (cc-rf). The line-integrated negative oxygen ion density amounts to between 2.5 × 1014 and 1015 m−2 depending on the oxygen pressure and rf power. Furthermore, the measured decay of the detachment signal reveals two modes of rf oxygen plasma characterized by different electronegativities. High electronegativity, α > 2, is associated with a low decay time constant of only a few microseconds, whereas in oxygen plasmas with low electronegativity, α < 1, the relaxation of electron density needs much longer with typical decay time constants of up to about 100 µs. The transition between the two modes shows a step-like characteristic and was observed at a specific rf power depending on the oxygen pressure. In the case of high electronegativity the electron density relaxation can be described by a simple 0D-attachment–detachment model, taking into consideration a constant density for positive ions and neutral oxygen species. Using the appropriate rate coefficients from the literature and the experimentally determined effective rate coefficients of first order kinetics, the evaluation of the attachment and detachment rates indicates the significant role of O2(a 1Δg) in the formation and loss of negative atomic oxygen ions.

The electron affinity of tungsten

The electron affinity of tungsten has been measured using laser photodetachment threshold spectroscopy in a collinear geometry. The electron affinity was determined to 6583.6(6) cm-1 by observing the onset of the process when W- ions in the $5d^56s^2$ 6S5/2 ground state are photodetached producing neutral W atoms in the $5d^46s^2$ 5D0 ground state. The measured value is in agreement with previous measurements and improves the accuracy by almost two orders of magnitude. Further, a photodetachment signal below the ground state photodetachment threshold was found, which indicates the existence of a bound excited state in W-.

O − density measurements in the pulsed-DC reactive magnetron sputtering of titanium

Using eclipse laser photo-detachment in conjunction with Langmuir probing, the density of O − ions in a reactive pulsed magnetron (100 kHz, 55% duty) plasma has been determined at different times during the pulse period at a set of positions along the centre line axis of the discharge. The magnetron was operated at a fixed average power of 400 W with an oxygen partial pressure of 10% of the total pressure 1.33 Pa. The results show the plasma is weakly electro-negative, with a negative ion-to-electron density ratio α up to a maximum of 0.63. During the plasma on-phase (at all chosen measurement positions) the O − density was found to reach a maximum directly after initiation of the voltage pulse decreasing weakly during the rest of the on-phase. On the transition from on-to-off phases of the pulse the negative ion density was found to fall (by 60% both close and far from the target but only 10% near the discharge centre), with the O − density remaining almost constant during the rest of the afterglow. The spatial structure of the O − density reveals a distinct peak 75 mm from the target close to but not at the position of the null in the magnetic field, falling by a factor of eight for increasing distances up to 30 mm both towards and away from the target. The highest O − density recorded at this position was 1 × 10 16 m − 3, at a time of 2.12 μs into the pulse. From a comparison between on- and off-phase densities and using an intuitive model of the plasma, the results indicate that most negative ions are created in the bulk plasma. The density of target-borne O − ions is estimated to be about 1 × 10 15 m − 3 varying little with position, possibly forming a beam-like structure.

Depletion of the excited state population in negative ions using laser photodetachment in a gas-filled RF quadrupole ion guide

The depopulation of excited states in beams of negatively charged carbon and silicon ions was demonstrated using collisional detachment and laser photodetachment in a radio-frequency quadrupole ion guide filled with helium. The high-lying, loosely bound 2D excited state in C− was completely depleted through collisional detachment alone, which was quantitatively determined within 6%. For Si− the combined signal from the population in the 2P and 2D excited states was only partly depleted through collisions in the cooler. The loosely bound 2P state was likely to be completely depopulated, and the more tightly bound 2D state was partly depopulated through collisions. 98(2)% of the remaining 2D population was removed by photodetachment in the cooler using less than 2 W laser power. The total reduction of the excited population in Si−, including collisional detachment and photodetachment, was estimated to be 99(1)%. Employing this novel technique to produce a pure ground state negative ion beam offers possibilities of enhancing selectivity, as well as accuracy, in high-precision experiments on atomic as well as molecular negative ions.

Negative ion density measurements in an inductively driven hydrogen discharge

The study presents experiments on the determination of the electronegativity and its axial variation in an inductively driven hydrogen discharge, performed by using the laser photodetachment technique. The results showing high electronegativity with nonmonotonic axial variations and a maximum next to the rf power deposition region provide an experimental indication that a design of an efficient source of negative hydrogen ions based on a single-chamber discharge might be possible.

Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O2/Ar gas mixtures (0–100%), applied powers (50–600 W) and total discharge pressures (2–25 mTorr). The measurements reveal that the O− ion dominates over with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200 W) and oxygen partial pressures (10% O2). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O− was determined to be ion–ion recombination with O+, and Ar+.In this study, the maximum negative ion density obtained was found to be 7.7 × 1015 m−3 at 200 W applied power, 25 mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, α = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.

Laboratory studies of the H2 production mechanism which led to the formation of the first stars

We have developed a novel apparatus to study associative detachment of H- and H forming H2. Beginning with an H- beam, we use laser photodetachment to neutralize a fraction of the beam. This generates self-merged, anion-neutral beams. Laboratory beam energies are in the keV range. Because the beams run co-linear, center-of-mass energies from the meV to keV range are achievable. Our measurements will help to resolve the nearly order of magnitude scatter in previously published results. Resolving this issue will have major implications for understanding the early universe chemistry which led to the formation of the first stars and protogalaxies.

Laser photodetachment on a high power, low pressure rf-driven negative hydrogen ion source

Powerful, low pressure negative hydrogen ion sources are a basic component of future neutral beam heating systems for fusion devices. The required high ion currents (>40 A) are obtained via the surface production process, which requires negative ion densities in the range of in the plasma region close to the extraction system. For spatially resolved diagnostics of the negative hydrogen ion densities, the laser photodetachment method has been applied to a high power, low pressure, rf-driven ion source (150 kW, 0.3 Pa) for the first time. The diagnostic setup and the data evaluation had to cope with the rf field (1 MHz), the high source potential during extraction (−25 kV) and the presence of magnetic fields (<10 mT). Horizontal profiles of negative ion densities and electron densities along 15 cm with a typical step length of 1 cm and a probe tip of 5 mm length show a broad maximum in the centre of the extraction region. The variation of a bias voltage applied to the plasma grid with respect to the source body yields a correlation between the detachment signals for the negative ion density and the electron density with the extracted ion and electron currents, respectively. The density ratio of negative hydrogen ions to electrons is in the range of , demonstrating that the negative ions are the dominant negatively charged species in these types of ion sources. Absolute negative ion densities are in good agreement with line-of-sight integrated results of cavity ring-down spectroscopy and optical emission spectroscopy.

Observation of theS1∕22metastable state inPt−

We have experimentally investigated the structure of the ${\mathrm{Pt}}^{\ensuremath{-}}$ ion using laser photodetachment threshold spectroscopy. The experiment was conducted using a collinear laser-ion beam apparatus, in which the residual atoms created in the photodetachment process were detected. A $p$-wave threshold was observed in the photodetachment spectrum at an energy of $6851(13)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We conclude that this onset originates from a photodetachment transition in which the initial state of ${\mathrm{Pt}}^{\ensuremath{-}}$ is the previously unobserved $5{d}^{10}6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}$ state and the final state of Pt is the $5{d}^{9}6s\phantom{\rule{0.2em}{0ex}}^{3}D_{3}$ ground state. The excitation energy of the $^{2}S_{1∕2}$ state is determined to be $10\phantom{\rule{0.2em}{0ex}}289(13)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This value can be compared with a multiconfiguration Dirac-Fock calculation performed by Th\o{}gersen et al. [Phys. Rev. Lett. 76, 2870 (1996)], which yielded an excitation energy of $11\phantom{\rule{0.2em}{0ex}}301\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our data show no indication of the presence of any other state of ${\mathrm{Pt}}^{\ensuremath{-}}$. We conclude that the structure of the ${\mathrm{Pt}}^{\ensuremath{-}}$ ion is now fully known.

Expanding hydrogen plasmas: photodetachment-technique diagnostics

The laser photodetachment technique in its combination with probe measurements is applied for the determination of the electronegativity and the concentration of the negative hydrogen ions in the expansion plasma volume of an inductively driven two-chamber plasma source. Both the implementation of the method and the obtained results are discussed in the study. The variations of the gas pressure and the applied rf power in the experiments are in the ranges p = (0.3–5) Pa and P = (200–700) W, respectively. The results for an almost constant—with the gas pressure—electronegativity and its increase with the applied power are in agreement with theoretical estimations. The obtained nonmonotonic variation of the concentration of the negative ions with increasing gas pressure correlates with the gas pressure dependence of the electron density in the expansion plasma region of the source.